Attempts have been made over the last several years to use antibodies in combination with cytotoxic agents, thereby seeking to effect selective action on target cells and to prevent or minimize the otherwise non-specific effect of cytotoxic agents. A review of the use of antibody-linked cytotoxic agents is provided in Ghose et al., J. Natl. Cancer Inst. 61, 657-676 (1978). As described in that publication, the methods of using a combination of antibodies and cytotoxic agents have ranged from covalent bonding using linkage-providing molecules, non-covalent binding, and simple mixing (see Table 1 of Ghose et al.).
With the advent of hybridoma technology and the accompanying availability of monoclonal antibodies, the feasibility of the antibody-cytotoxic agent approach has brightened considerably due to the ability potentially to precisely direct the complex to the intended target cell population.
To date, the concept of using antibodies in conjunction with biologically active agents has been limited to those agents which have inherent and nonspecific cytotoxic activity, and the degree of specificity has been dependent upon the limiting effect, if any, imposed by the accompanying antibody. Thus, a variety of recognized cytotoxic agents, including, for example, diphtheria toxin [Gilliland et al., Proc. Natl. Acad. Sci. USA 77, 4539-4543 (1980)]; ricin [Krolick et al., Proc. Natl. Aca. Sci. USA 77, 5419-5423 (1980)]; subunit A of ricin (U.S. Pat. No. 4,350,626); and gelonin [Thorpe et al., Eur. J. Biochem. 116, 447-454 (1981)], have been covalently coupled to antibody molecules.
This approach, of course, suffers from certain limitations. First, as noted, the agent carries inherent and indiscriminate cytotoxic activity. Subject to whatever degree of cell-directing specificity the antibody may provide, the cytotoxic agent may otherwise indiscriminately attack any cell it can approach. Secondly, although the activity of the cytotoxic agent is indiscriminate, it is dependent upon and limited by the ability of the agent to become internalized by the target cell.
In view of these limitations, it appeared ideal to seek to develop a conjugate, the action of which is not dependent upon an agent's innate and indiscriminate cytotoxic properties, but which instead is capable, in conjunction with the cell-fixing properties of the antibody, of utilizing one of the host's own cytotoxic mechanisms, namely, complement. This approach circumvents non-specific toxicity and avoids the need for internalization by the target cells of the cytotoxic agent.
It is to such a composition that this invention is directed. Briefly, this invention is directed to a composition comprising a molecule having binding affinity to a surface structure of a cell and a molecule having activity as a structural subunit of the stable and control resistant C3/C5 convertase. A preferred composition of this invention is one comprising an antibody directed to a cell surface constituent and coupled to cobra venom factor (CVF).
Cobra venom factor (CVF) is a glycoprotein having a molecular weight of about 140,000 present in the venom of the cobra Naja naja. CVF is the structural subunit of a C3/C5 convertase of the alternative complement pathway. It appears to be immunochemically [Alper et al., Science 191, 1275-1276 (1976)] and structurally [Vogel et al., Fed. Proc. Fed. Am. Soc. Exp. Biol. 40, 1020 (abst.) (1981)] related to human C3 and C3b. CVF exhibits a C3b-like function when added to mammalian serum in that, together with factor B and in the presence of factor D and Mg.sup.++, it forms a soluble C3/C5 convertase (CVF,Bb) that efficiently initiates assembly of the membrane attack complex (MAC) of complement.
Although CVF exhibits the above actions, it is not cytotoxic per se. Instead, it has been shown, upon injection into rabbits, that CVF, upon complexing with a plasma constituent, leads to inactivation and depletion of C3 [Cochrane et al., J. Immunol. 105, 55-69 (1970); see also, Vogt et al., Hoppe-Seyler's Z. Physiol. Chem. 355, 171-183 (1974)]. However, when coupled with an antibody directed to a cell surface constituent, the complement-activating action of CVF is effective in activating the host's own cytotoxic complement mechanisms specifically against the antibody target cell.